1. Field of the Invention
The present invention is directed to making optical elements, particularly refractives, using gray level or scale masks. More particularly, the present invention is directed to making gray level masks with a high resolution and which can be used for fabricating multiple arrays of refractive elements simultaneously. The present invention is also directed to transferring patterns of gray level masks onto a photoresist using contact or proximity printing. The photoresist is then used to make the desired optical element. The gray level masks can be used repeatedly for making optical elements.
2. Description of Related Art
One conventional technique for forming a refractive element includes forming a structure in photoresist by patterning and melting a photoresist layer on a glass substrate. This melting of the photoresist generates spherical surfaces. Such a technique is disclosed, for example, in an article by O. Wada, "Ion-Beam Etching of InP and it's Application to the Fabrication of High Radiance InGAsP/InP Light Emitting Diodes", General Electric Chemical Society, Solid State Science and Technology, Vol. 131, No. 10, October, 1984, pp. 2373-2380.
However, this technique is limited to special shapes and can only provide spherical contours using a small positive photoresist layer. Further, the refractive elements are produced by ion milling the resist structure and the glass substrate. The ions first mill the resist. Once the resist is removed in a certain region, the ions mill the glass substrate. In this manner, the resist structure is transferred to the glass substrate, thereby forming the refractive element.
A varied exposure pattern in a photoresist can also be generated by directly exposing the photoresist with a raster-scanned laser or electron beam. However, in this case, no mask is created. Each element must be written one at a time, with no benefit of economies of scale. It is desirable to create a gray scale mask that can be reused multiple, for example, thousands, of times to make thousands of wafers.
In attempting to overcome these limitations, an exposure mask for fabricating microlenses was developed and disclosed, for example, in U.S. Pat. Nos. 5,480,764 and 5,482,800 to Gal et al. and in an article by W. W. Anderson et al. "Fabrication of Micro-optical Devices" Conference on Binary Optics, 1993, pp. 255-269. According to this technique, known as half-toning, the mask is created by constructing a plurality of precisely located and sized openings, the frequency and size of these openings producing the desired gray scale effect.
In this technique, each pixel is divided into sub-pixels and each sub-pixel is sub-divided into gray scale resolution elements. The smallest features of this mask, i.e., the gray scale resolution elements, are binary. These gray scale resolution elements are either open or closed, i.e., on or off. The size of a mask opening for that sub-pixel is selected using sub-pixel location information and height or thickness information for that location. The size of the mask opening in each sub-pixel provides a gray scale resolution depending upon the number of resolution elements incorporated with that mask opening.
However, in using such a mask, in addition to the strict fabrication requirements, the mask is used with a stepper, i.e., the pattern of the mask is effectively reduced in size when exposing the resist layer. This reduction is required, since the gray scale resolution elements are binary, they must be blurred in order to present the desired gray scale effect so that the gray scale resolution elements no longer appear to be distinct holes. This leads to the mask having to be a number of times larger than the actual element. Thus, for simultaneously producing many elements, the mask will soon become impracticably large. Additionally, steppers are very expensive equipment.
Further, due to the required reduction, the point-spread function is larger than the image of the smallest opening in the mask. This blurring allows the mask to form a gray level pattern in the photoresist, but the large size of the point spread function results in a decreased resolution. As a larger number of gray levels is required, the larger the point spread function required, i.e., the more blurring required. Thus, this technique becomes impractical for a large number of gray levels.